Trimethylaluminum (TMAL) has the potential of becoming a significant commercial product with many applications such as a polymerization co-catalyst, a substrate for production of compound semiconductors, a starting material to produce trimethylgallium and methylaluminoxanes, and so forth.
Several methods have been known for trimethylaluminum synthesis such as the "Magalium" process (U.S. Pat. No. 2,744,127), the cryolite process (U.S. Pat. No. 2,839,556), the phenyl sodium process (Adv. Inorg. Chem. Radiochem. 7, 269, 1967), the Grignard route (K. Ziegler, Organometallic Chemistry (ACS Monograph No. 147) p. 197, 1960), and so forth. The alkali reduction method has been the process most extensively used in the industrial as well as the laboratory scale preparation.
The overall reaction is described by Grosse and Mavity, J. Org. Chem., Vol. 5, pp. 106-121 as follows: EQU 3 Me.sub.2 AlCl+3 Na.degree..fwdarw.2 Me.sub.3 Al+Al.degree.+3 NaCl
This method involves reacting dimethylaluminum chloride (DMAC) with molten metallic sodium which is dispersed in a suitable hydrocarbon solvent. TMAL produced can be isolated by vacuum distillation. The yield obtained by this method is low because the resulting sodium chloride and metallic aluminum deposit a coating on the surface of sodium metal. This effect which can cause severe reaction fouling may be magnified by the side reaction between TMAL and excess sodium to produce metallic aluminum and insoluble sodium tetramethylaluminate. EQU 4 Me.sub.3 Al+3 Na.degree..fwdarw.3 Me.sub.4 AlNa+Al.degree.
The sodium reduction of DMAC has been studied in glassware using several different solvents (Soltrol 160 solvent, Tetralin solvent, n-heptane, n-decane, and dodecane). The yields ranged from 47% to 85% and the product was contaminated with unreacted DMAC even though excess sodium was used.
Experimental work has also been conducted earlier whereby DMAC was reacted with sodium as the sole reducing agent in a larger, pilot plant reactor. Although a yield of about 70% was obtained, a large excess of sodium and a very long reaction time was required for complete reduction of DMAC. In addition, reactor fouling due to formation of by-products was severe.
From these experiments, it can be concluded that there are numerous drawbacks to this route: The yield is low; sodium chloride and metallic aluminum by-products and unreacted sodium form a considerable amount of lumps which are difficult or impossible to transfer out of reactor causing problems in waste disposal and safety; the reduction does not readily go to completion resulting in a product which is contaminated with unreacted DMAC and; excess sodium is required for a complete reduction to take place at a reasonable rate. A larger excess of sodium is undesirable due to yield loss (reaction 2) and reactor fouling. This side reaction reduces the amount of TMAL produced and thus diminishes the final yield; reaction time is long; and the use of high boiling solvents gives reduced yield and purity (unreacted DMAC contamination in distilled product). But high boiling solvents are desirable because they remain in the vessel after TMAL recovery to fluidize the solid by-products, NaCl and Al.degree.. In contrast, lower boiling point solvents codistill with TMAL to contaminate final product.